1. Field of the Invention
This invention relates generally to guyed construction techniques, and, more particularly, to techniques for anchoring and for reinforcing the anchoring of guyed and additionally guyed towers.
2. Description of Related Art
Towers are widely used in many industries, including television transmission, radio communication, cell phone communication, wind turbines, and power transmission, to name a few.
Some towers, known as “guyed towers” or “additionally guyed towers,” rely on guy wires to maintain or assist in maintaining the towers in a vertical orientation. Generally speaking, these towers include a vertical main body, or “mast,” that stands on one end atop a base, which is generally concrete. Guy wires attach to the mast along its length, extend down and away from the mast, and attach securely to the ground using anchors. Most guyed towers are triangular in cross-section, and a minimum of three guy anchors are typically provided and are spaced apart by approximately 120-degrees to provide a stable base for holding the mast vertically. Often, guyed towers require three, six, or more guy anchors with multiple guy wires originating from different vertical levels of the tower attached to each guy anchor.
The term “guyed towers” describes towers whose masts have no independent means of support. They rely entirely upon guy wires to hold them upright. By contrast, the term “additionally guyed towers” describes towers that are essentially free standing, although they require guy wires to provide reinforcement and stability.
FIG. 1 shows a conventional guy anchor 100 for an erected tower. As shown in this example, four guy wires 110 originating from the tower's mast attach to an anchor head 114. The guy wires 110 are generally composed of steel or some other high tensile strength metal. A shaft 116 extends from the anchor head 114 and into the ground 124. Typically, the anchor head 114 and shaft 116, which are also generally made of steel, are provided as a single unit, with the shaft 116 permanently welded to the head 114. The distal end of the shaft 116 is typically buried in a steel-reinforced mass of concrete 118, also known as a “dead-man.” The weight of the dead-man 118 and the earth above it holds the shaft 116 securely in place, even in the presence of large forces on the tower due to wind and precipitation.
The typical guy anchor assembly 100 may also include turnbuckles 112. One turnbuckle 112 is generally provided for each guy wire 110. The role of the turnbuckles 112 is to fine-tune the tightness of each guy wire 110.
To prevent damage due to lightning strikes, the guy wires 110 are each electrically connected via a conductive cable 120 to a ground spike 122. The ground spike 122 is typically made of copper. The cable 120 and ground spike 122 form a low impedance path to ground. This arrangement is designed to conduct high current surges away from the shaft 116, thereby preventing damage to the shaft which could otherwise compromise the mechanical stability of the tower.
As is known, the shafts 116 of the guy anchors typically corrode over time. Guy shaft corrosion primarily affects the area of the shaft exposed to soil, i.e., underground but outside the region encased in the dead-man 118. Corrosion may be galvanic in nature, with the steel guy shaft forming a battery cell with the more noble copper ground spike 122. Corrosion may also be electrolytic in nature, or may be caused by other factors.
Over several years, corrosion may lead to a significant loss of material from the anchor shaft 116, which, under the tensile forces transmitted through the guy wires, can result in a separation of the guy anchor shaft from the dead-man and a consequent catastrophic collapse of the tower.
The cost of replacing a collapsed 120 meter wireless guyed tower is estimated to be approximately $400,000. In addition, tower collapse poses a great risk to human life and property in the vicinity of the tower.
Owners and operators of guyed towers have developed aggressive remedial measures to prevent guy anchor failure. These include the following:                1. Inspecting the anchor shafts. This technique involves excavating around an existing anchor shaft to visually ascertain the status of the anchor shaft. Since the complete anchor shaft must typically be inspected, excavation is generally all the way to the dead-man 118. Removing earth above the dead-man temporarily weakens the guy anchor, and measures must be taken to retain the anchor in the ground as inspection proceeds.        2. Installing a new dead man anchor in front of the corroded anchor. This approach requires relocating the existing guy wires from the corroded anchor shaft to the new one.        3. Installing a new anchor behind the corroded anchor. Because distance to the tower mast is increased, this approach generally requires replacing all the guy wires, as they will be too short to re-attach to the new guy anchor. The additional space needed for the modified tower may require the tower owner to acquire new property or easements.        4. Installing a new drilled pier anchor offset to one side of the corroded anchor. This approach requires relocating the existing guy wires from the corroded anchor shaft to a new one. Towers with pinned bases may be caused to rotate to re-align themselves with the new anchors. Rotating the towers can sometimes be hazardous, and any antennas on the towers will generally need to be realigned. In addition, some towers have fixed bases and cannot freely rotate, in which case relocating the guy wires to new anchor heads can place additional stresses on the towers, which can lead to other problems.        